Television broadcasts may help carriers put tiny cellular base stations into …

Rosum would probably like to thank the television broadcasting industry for putting up fixed transmitters all over the world with accurate-enough timing information in publicly broadcast data to let the firm create GPS-like results without a satellite in sight.

Rosum's focus in the world of GPS is not to replace GPS—a system partly architected by one of the company's founders—but to supplement GPS in places where satellite signals can't be received, generally indoors, as well as urban canyons.

Rosum is not a new firm, but the time may have arrived for its technology, which allows the company to provide either a GPS-like fix on a location by itself or to assist an inexpensive GPS module that is receiving extremely weak satellite data.

Todd Young, the company's vice president of marketing, told Ars that the company's focus is on bringing "timing and location [information] to in-building and urban and even dense canopy areas in which GPS just doesn't work." That's a neat overlap with where cellular carriers are trying to place femtocells, compact cell base station intended to improve indoor coverage. (See "Study: consumers likely to greet femtocells with yawns," among other articles.)

The company is also looking at television viewed on mobile handsets, an already large and rapidly growing service in Japan and South Korea. A North American standard is moving towards adoption, too. A handset with a digital TV receiver could supplement GPS in the same way that WiFi is being used by Skyhook Wireless to provide location information in difficult locations for the iPhone and other smartphones.

But femtocells are the immediate target because femtocells have a location requirement for both operation and emergency calling in North America. Carriers in many nations have unique groups of spectrum licenses in each area they serve. "Whoever's operating that network has to have a license to operate a wireless network in that geographic location," Young explained, "So you have to check and make sure you're in an area that's covered by that license."

Further, femtocells need to prevent their use outside a given country to avoid stepping on licenseholders there. All femtocells sold to date include a GPS receiver—and a 30-foot GPS antenna. Running 30 feet of cable is an ugly task on any day—ask anyone with poor XM or Sirius reception—but it may be particularly hard when trying to place a femtocell next to a broadband modem.

If a femtocell can't get a GPS lock, it can't be used, which means an unhappy customer who thought the carrier was offering a solution to coverage that's been pulled out from under them, and a return of an expensive item to the carrier with all the cost in handling that entails. Improving the GPS capabilities of a femtocell could be a make-or-break difference in carriers convincing their subscribers to buy into the idea.

Rosum can provide a much simpler solution for location data because they rely on TV broadcasts, which typically use far lower frequencies than cellular networks, and far higher power. The benefits "are not just lower attenuation from building materials—walls and ceilings and floors and such—it's that the broadcast power is so much higher, and they're roughly 12,000 miles closer than the satellite," Young said.

Young noted that there's about 30 dB (decibels) of signal margin for GPS, and about 80 dB for the part of the television signal that Rosum requires (a TV needs about 30 dB more than Rosum to display a picture). A single concrete floor blocks about 30 dB, Young said.

The accuracy that Rosum can deliver is on the order of a conventional assisted GPS system, Young said. With analog TV signals, their margin is worse, about half as good as digital TV. With DTV, "There are a couple of clocks in the signal chain that drive the timing of the signals. One is where the MPEG stream is encoded; the other is where the signals have converted to its broadcast frequency," he said.

Rosum so far has only one publicly announced femtocell agreement with 2Wire, a router and electronics maker. Young said, however, that the company is in active talks with just about every femtocell company.

Young also pointed out that femtocells may be used by carriers to fulfill Enhanced 911 regulations, which require carriers to provide a certain degree of accuracy to a certain percentage of subscribers in a given area: calls relying on handset location must deliver information accurate to within 50 meters 2/3rds of the time, and within 150 meters, 95 percent of the time.

With an accurate position assigned to a femtocell, which is likely to move very little at all, a carrier might be able to improve its E911 compliance dramatically with a large femtocell deployment into subscribers' homes without making other changes to their network or location technology.

Young and others see a larger future for femtocells than just relying cell calls or 3G data. Rather, the femtocell could become part of a home hub for wireless communication. A ZigBee (802.15.4) home monitoring system could rely on the femtocell for both relaying data to a locally network device, and providing remote access or monitoring over a cell network without loading down a cell provider's system. (ZigBee is starting to appear in home sensors and alarm systems.)

For the future, Young is looking at the mobile broadcast television market, which has tens of millions of users in Japan and South Korea. For a GPS offering, "Once the enabling hardware is on the phone, the marginal cost is zero, the footprint is zero, it's just software," he said.

Where WiFi positioning systems like Skyhook Wireless require constantly updated databases of WiFi networks, "TV towers don't move," Young said. There are just a few thousand U.S. locations the company needed to characterize accurately. A public FCC database let them get close, and their own work added precision to the numbers the agency records.

Also, as opposed to WiFi, TV signals can penetrate into many environments where WiFi networks won't be operating or won't provide a close enough location fix. Young said that in a recent test with a femtocell manufacturer, Rosum was able to get an accurate GPS fix in 8 out of 8 cases in which two assisted GPS systems managed just 1 of 8 results. This included the Montgomery Street BART station in San Francisco, a floor beneath the street level, where Young says Rosum's system provided 40 to 50 meters fixes.

Rosum's solution is certainly unique—at least at the moment. The firm has a couple of niche markets that could explode, and mobile television may have the most interesting implications for those of us in the U.S.

So, this won't work in when the receiver is moving, like in a car, right? Last I checked, DTV doesn't work "at speed."

Rosum isn't looking to display a picture, but is extracting timing signals, so I expect that the ability to do that while in motion isn't a real problem, especially outdoors where the signal strength will be enormously higher for their purposes.

Originally posted by backbay13:Yeah but don't most cities and towns (like las vegas) only have one television tower, usually on a mountain or highrise. Can you triangulate from one tower? Or am I missing something?

I show at least three separate antenna facilities in/around Las Vegas, with active licenses. More generally, one cannot determine location using only one antenna and one signal, but this service is apparently seen more as a supplement rather than as a stand-along location method.

I've briefly taken a look at the Rosum solution in the past. It is even stranger than you might think. Because the television signals don't have to be traceable to a good clock source like GPS is, Rosum actually puts a receiver in an area of interest and figures out the frequency offset from GPS accuracy for each of the television signals in the area. Then this information has to get sent to all off the femtocells so they can derive an accurate signal.

Kind of convoluted. I'd prefer a low-signal GPS solution like SigNav (if it actually works inside buildings), or timing over Ethernet like 1588 (though this only gives you time, not location).

Well, at least according Wikipedia, the issue is that ATSC is "susceptible to changes in radio propagation conditions" ( http://en.wikipedia.org/wiki/ATSC#Comparison ), specifically because of the design of the single-frequency, distributed transmission system ( http://en.wikipedia.org/wiki/D...tem#Technical_issues ). Recent receivers do OK at coping with the terrible multipath performance of ATSC, but a changing multipath environment like you would get in a moving car is still infeasible.